Conventional lithography systems include, among other things, an illumination system to produce a uniform intensity distribution of a received laser beam. It is desirable that the resulting illumination be as uniform as possible and that any uniformity errors be kept as small as possible. Illumination uniformity influences the ability of an illumination system to produce uniform line widths across an entire exposure field. Illumination uniforrmity errors can significantly impact the quality of devices produced by the lithography system.
Techniques for correcting uniformity include correction systems that have multiple correction elements in the form of rectangular plates inserted from opposite sides of an illumination slot. The shape of these correction elements limits their ability to correct uniformity errors across a wide range of illumination systems and/or illumination modes. In addition, because of the finite nature of these correction elements, ripples are induced in the uniformity profile.
Therefore, what is needed is a uniformity correction system that compensates for uniformity errors across a wide range of illumination systems and illumination modes.
Furthermore, when correcting uniformity it is important to control the uniformity of the illumination pupil because it also has significant impact on lithography. To minimize illumination pupil non-uniformity induced by uniformity correction, the correction should be performed close to a reticle conjugate plane. However, uniformity correction techniques which use discrete segments produce increased uniformity ripple the closer they are to a reticle conjugate plane. Therefore, what is further needed is a uniformity correction system that can correct for uniformity ripple near a reticle conjugate plane.
The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers can indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number may identify the drawing in which the reference number first appears.